In a known type of furnace, product is conveyed through a heating chamber of a furnace on a conveyor belt. The belt travels over the surface of a hearth, which is typically formed by shallow metal pans called hearth plates. A furnace muffle surrounds the hearth plates and the belt to define the heating chamber. See FIG. 1, in which the belt is omitted and a portion of the muffle is shown cut away to better illustrate the hearth plates.
This type of conveyor furnace is used for a variety of applications, including stainless steel brazing. Stainless steel brazing requires a high temperature and a low dewpoint inside the furnace. In this application, copper wire is wrapped around a joint. The product with the joint is passed through the furnace, melting the copper and brazing the joint. The melting temperature of copper is approximately 1080.degree. C., so the temperature within the muffle must be in excess of 1100.degree. C. and is typically in the range of 1120 to 1130.degree. C. The oxygen level must also be very low, typically measured by a dewpoint of -70.degree. F. or less in a hydrogen atmosphere. Because oxygen in a hydrogen atmosphere reacts with the hydrogen to form water, the oxygen level in such a furnace is typically determined by measuring the dewpoint in the furnace.
In this type of furnace, there is continuous friction between the belt and the hearth. Hearth materials vary, but the overwhelming choice of hearth material is metal and for higher temperature applications is typically a nickel-chrome (Ni--Ch) alloy. This material is also used for the belt. This material is able to withstand the temperatures in the furnace while retaining sufficient strength to support the product without failure. The coefficient of friction between the belt and the hearth ranges from 0.3 to nearly 0.6 when the temperature is above 1100.degree. C. A high coefficient of friction causes serious drag and wear life problems.
Also, in furnace atmospheres where oxygen is present, slight oxidation occurs on the belt and hearth plates, providing a protective coating against wear and grinding. In furnace atmospheres with very low dewpoints and at high temperatures, however, no or minimal oxidation occurs, and metal is often transferred from the belt onto the hearth. These metal deposits on the hearth create bumps and burrs that cause severe drag and wear problems against the traveling belt.